The present invention relates to a ceilometer or a device for measuring height of clouds of the type which includes a transmitter for transmitting light pulses, a receiver including a photodetector for receiving light pulses reflected from clouds, gating means for passing output signals from the receiver during time gaps or time slots following each other at transmitter frequency, an integrator connected to the gating means for interpreting the passed output signals over a time interval covering a plurality of time slots having the same phase shift relative to the transmitted light pulses, and a memory for storing the integrated signal values assigned to a height value determined by the phase shift of the time slots.
Devices of this kind are known from the Swiss Patent No. 628,139 and from the German Patent No. 2,924,490. The prior art devices indicate as a cloud height or heights those height values which are associated with the largest signal values stored in the memory. They operate without problems provided that no precipitation occurs. However, when rain or snow falls, then light pulses emitted by the transmitter are reflected not only by clouds but also by rain drops or snow flakes. Signal values produced in response to the precipitation may be greater than signal corresponding to a cloud height. Accordingly, in prior art devices there is the risk that during precipitation an incorrect cloud height is indicated.